Lubricating lipids in hydrogels

doi:10.1126/science.abd3831

GSTDTAP > 气候变化

DOI	10.1126/science.abd3831
	Lubricating lipids in hydrogels
	Tannin A. Schmidt
	2020-10-16
发表期刊	Science
出版年	2020
英文摘要	Hydrogels are hydrophilic polymer chain networks that can absorb large quantities of water or biological fluids. In many of their biomedical and other applications ([ 1 ][1]), hydrogels slide against another surface and must be well lubricated. Hydrogel lubrication is normally provided by fluid interfacial layers ([ 2 ][2]), but the long-lasting lubrication of articular cartilage in our joints ([ 3 ][3]–[ 5 ][4]) is partially the result of nonfluid, highly hydrated head groups of exposed phosphatidylcholine lipids ([ 6 ][5]). On page 335 of this issue, Lin et al. ([ 7 ][6]) mimic and modify this mechanism in various synthetic hydrogels by incorporating small concentrations of lipids to create a self-renewing, molecularly thin lipid-based boundary layer. The results are striking, with substantial reduction of friction (and wear) observed by as much as a factor of 100 relative to lipid-free hydrogels. Moreover, the effect remained even when gels were dried and rehydrated. Articular cartilage is the lubricious, load-bearing tissue at the end of long bones in synovial joints that facilitates both low-friction and low-wear articulation. Although the low friction of cartilage is attributed in large part to fluid pressurization that can bear a substantial fraction of the load, lubricant molecules also play an important role in mediating friction, especially with surface-to-surface contact in the boundary mode of lubrication. Synovial fluid constituents, which include hyaluronan, lubricin, and surface-active phospholipids, all can and do contribute to the boundary lubrication of cartilage. Indeed, their independent and synergistic function has been an active area of research and discussion ([ 3 ][3], [ 8 ][7]–[ 11 ][8]). Although these molecules have been studied in the context of hydrogel lubrication in the past, Lin et al. found that embedding lipids in microreservoirs within hydrogels creates a material with self-renewing boundary lubrication. ![Figure][9] Cartilage-inspired, lipid-based boundary-lubricated hydrogels Liu et al. incorporated small concentrations of lipids to create a self-renewing, molecularly thin lipid-based boundary layer. This discovery has potential applications in numerous fields relevant to biology and medicine. GRAPHIC: KELLIE HOLOSKI/ SCIENCE Biotribology is the study of lubrication, friction, and wear when applied to biological systems or natural phenomena ([ 12 ][10]). Friction coefficients are systems variables that depend on test variables, surfaces, and geometries. With that dependence in mind, Lin et al. provide compelling data demonstrating the robust and effective lipid-based boundary lubrication in their hydrogels. They examined a number of different hydrogels, both biological and synthetic polymers [some quite different from the commonly used poly(hydroxyethylmethacrylate), pHEMA], and showed effective reduction of friction with the lipid incorporation. They also tested different test countersurfaces other than steel and again found effective friction reduction. Finally, they examined a wide range of sliding velocities (more than three orders of magnitude). A near-constant friction coefficient provided evidence that boundary-mode lubrication is operative. This last result highlights the importance of incorporating lipids into a hydrogel to provide self-renewing boundary lubrication. The authors further demonstrate that sliding is the driving mechanism for lipid-layer formation on the gel surface from the microreservoirs of lipid vesicles, and that the self-renewal of this boundary lubricating layer occurs as the hydrogel wears (ever so slightly) under friction (see the figure). Moreover, lubrication by external application of lipids, either through soaking and washing hydrogels or by their presence in solution, is far less effective than when the lipids are bulk-incorporated. Lipids from the surrounding solution had poorer access to the gel/countersurface interface relative to those incorporated into vesicles. Also, the incorporation of lipid vesicles did not alter the mechanical properties of the hydrogel, which may be relevant in various applications. Additionally, the concentration of incorporated lipids could be altered, which allowed tuning of the lubricating ability and duration of the hydrogels. A potential critical consideration regarding the utility of these hydrogels is that wear is required for these hydrogels to function in their self-renewing manner. Although the authors demonstrate that this wear is minimal and that the materials can sustain many cycles, this parameter could affect longevity and would need to be taken into consideration for different applications. Finally, one amazing feature of these hydrogels that could potentially open up many applications is that they retain their self-sustaining lubrication even after being fully dried in an oven to 60°C and rehydrated. Indeed, this robust property has implications for storage of hydrogels and potentially for their utility in harsh conditions that would not be possible for many other materials. Lin et al. have demonstrated a simple yet effective way of creating self-lubricating hydrogels through incorporation of lipids, with minimal effect on the bulk mechanical properties. 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领域	气候变化 ; 资源环境
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引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/299327
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Tannin A. Schmidt. Lubricating lipids in hydrogels[J]. Science,2020.
APA	Tannin A. Schmidt.(2020).Lubricating lipids in hydrogels.Science.
MLA	Tannin A. Schmidt."Lubricating lipids in hydrogels".Science (2020).